fix(attachments): download/view attachments on cross-account All-Mail messages

Blobs are scoped per JMAP account, but the attachment download/preview path
always used the active account's client and accountId. Opening a message from a
different account in the unified / All-Mail view and downloading (or previewing)
an attachment therefore 404'd against the active account.

Route the blob fetch to the message's source instead:
- resolveBlobSource() picks the owning login's client
  (getClientForAccount(sourceClientAccountId)) and the owner accountId
  (sourceAccountId) for delegated/shared blobs, in the unified view;
- handleDownloadAttachment + the attachment-preview handlers use it;
- downloadBlob / fetchBlobAsObjectUrl / fetchBlobArrayBuffer gain an accountId
  param (getBlobDownloadUrl/fetchBlob already had one).

Adds 10-attachments: an attachment on another account's All-Mail message
downloads with the correct bytes (verified to fail without the routing).

integration/tests/10-attachments.spec.ts

@@ -0,0 +1,77 @@
+import { test, expect } from '@playwright/test';
+import { ACCOUNTS } from './helpers/config';
+import { sendMail } from './helpers/smtp';
+import { JmapClient } from './helpers/jmap';
+import {
+  login,
+  addAccount,
+  switchAccount,
+  seedSettings,
+  folderRow,
+  openFolder,
+  emailItem,
+  expectEmailVisible,
+  forceSync,
+} from './helpers/app';
+
+/**
+ * Attachments on a message that belongs to a *different* account, opened from
+ * the cross-account All-Mail view. Blobs are account-scoped, so downloading one
+ * must route to the owning account's client + accountId — otherwise it 404s
+ * against the active account (the reported bug).
+ */
+const { alice, bob } = ACCOUNTS;
+const ATT = { filename: 'report.bin', contentType: 'application/octet-stream', content: 'hello-attachment-content-12345' };
+
+test.describe('Cross-account attachments', () => {
+  test.beforeEach(async () => {
+    for (const a of [alice, bob]) {
+      const j = await JmapClient.connect(a.email, a.password);
+      await j.reset();
+    }
+  });
+
+  test('an attachment on another account\'s All-Mail message downloads correctly', async ({ page }) => {
+    const subject = `IT attach ${Date.now()}`;
+    // Deliver a message with an attachment to bob.
+    await sendMail({ from: bob.email, authPass: bob.password, to: bob.email, subject, body: 'see attachment', attachment: ATT });
+
+    // Cross-account All Mail + always download attachments (don't preview).
+    await seedSettings(page, {
+      enableUnifiedMailbox: true,
+      enableCrossAllView: true,
+      unifiedCrossAccount: true,
+      includeGroupInUnified: true,
+      mailAttachmentAction: 'download',
+    });
+
+    // Make alice the active account, with bob added, so bob's message is
+    // genuinely cross-account when opened.
+    await login(page, alice);
+    await addAccount(page, bob);
+    await switchAccount(page, alice.email);
+    await forceSync(page);
+
+    // Open the All-Mail view and bob's message.
+    await expect(folderRow(page, { name: '__cross_all__' }).first()).toBeVisible();
+    await openFolder(page, { name: '__cross_all__' });
+    await forceSync(page);
+    await expectEmailVisible(page, subject);
+    await emailItem(page, subject).first().click();
+
+    // The attachment chip is present; clicking it downloads the blob from bob's
+    // account (pre-fix this 404s against alice and no download fires).
+    const chip = page.locator(`[data-testid="attachment"][data-attachment-name="${ATT.filename}"]`).first();
+    await chip.waitFor({ state: 'visible', timeout: 15000 });
+
+    const [download] = await Promise.all([
+      page.waitForEvent('download', { timeout: 15000 }),
+      chip.click(),
+    ]);
+
+    const stream = await download.createReadStream();
+    const chunks: Buffer[] = [];
+    for await (const c of stream) chunks.push(c as Buffer);
+    expect(Buffer.concat(chunks).toString()).toContain(ATT.content);
+  });
+});